This invention relates to a perpendicular magnetic recording medium useful as a high-density magnetic recording medium.
The perpendicular magnetic recording method is excellent as a high-density recording method. Many developments have heretofore been made and are currently under way by a variety of researchers on magnetic heads and magnetic recording media, both of which are important to practice the above method.
Those developed as perpendicular magnetic recording media to date may be divided roughly into the following two groups, depending on their production processes:
(1) Those obtained, each, by dispersing ferromagnetic particles in a resin or the like and subjecting the resultant dispersion to a magnetic field upon its coating and drying so as to orient the ferromagnetic particles in the magnetic field; and
(2) Those obtained, each, by forming a sputtered or vaccuum-deposited film with axes of easy magnetization oriented and grown in a direction perpendicular to the surface of the film.
According to the process (1), a coating formulation containing ferromagnetic particles dispersed at random in a binder such as resin is solidified while causing the ferromagnetic particles to orient in a magnetic field, thereby obtaining a recording medium such as magnetic tape or magnetic disk. Various problems are however encountered in causing such ferromagnetic particles to orient in the direction parallel to the substrate plane. Namely, the process, (1) is accompanied by such drawbacks that the surface of each resultant coating film tends to become unstable and hence rough due to magnetic poles that appear on the surface, and the orientation of the magnetic particles is reduced because when the external magnetic field is removed after the magnetic particles have been oriented once, a magnetic field is developed in the direction opposite to the orienting magnetic field due to magnetic poles remaining in both sides of the coating film, particles positioned at angles relative to the thus-developed magnetic field are subjected to torques and hence tend to orient in the direction parallel to the substrate plane.
On the other hand, the process (2) makes use of an alloy composition, which is easy to orient along the C-axis, upon forming a coating film by sputtering or evaporation method. At the same time, the temperature of the substrate is raised to obtain the coating film having an easy magnetization axis perpendicular to the substrate plane. There are however inherent limitations imposed on the alloy composition and film-forming conditions. The process (2) cannot always make the axis of easy magnetization of a ferromagnetic material orient in the perpendicular direction. In addition, the requirement for the raised substrate temperature has resulted in a shortcoming that a polymer material having a low softening point cannot be used as a material for the substrate. Besides, in many instances, resultant coating films have columnar structures extending in the directions perpendicular to the planes of the coating films and thus have poor flexibility.